1. Field of the Invention
This invention relates to shaping glass sheets and in particular to reducing the residual stresses in glass sheets formed on shaping rails by sag bending.
2a. Technical Considerations
Bent glass sheets are commonly used as glazing closures in vehicles such as automobiles and the like. For such applications, the glass sheets must be bent to precisely defined curvatures dictated by the configuration and outline of the openings in the vehicles in which the bent sheets are to be installed as well as the overall vehicle styling. At the same time it is important that the bent sheets meet stringent optical and strength requirements.
It is common practice to shape a glass sheet on a bending mold having contoured shaping rails that support an initially flat glass sheet and convey it through a heating lehr. As the temperature of the supported glass approaches its heat softening temperature, it begins to sag under the force of gravity and conform to the contours of the shaping rails on the mold. If required, the bending mold may include hinged end sections to help facilitate bending portions of the glass sheet to sharp curvatures that could not normally be obtained through normal sag bending techniques. Where windshield glass is being shaped, two overlaying glass sheets, or doublets, may be conveyed through the lehr on the bending mold and shaped simultaneously.
After shaping, the glass sheet is cooled. When the glass is to be annealed, i.e. cooled slowly through its annealing range, as would be the case if the glass is to be used to fabricate a laminated windshield, it is common practice to cool the glass while it remains on the bending mold. It has been found that selected components of the bending mold, and in particular the mold shaping rails, support framing, ballast and/or heat shields function as heat sinks during the heating and sag bending operation and reradiate absorbed heat during the cooling operation. As a result, portions of the glass closer to these components of the mold cool more slowly than the surrounding portions of the glass. The non-uniform cooling produces localized high tensile stresses in those portions of the glass that cool at a slower rate which in turn makes these highly stressed areas more susceptible to breakage from impact. This effect is particularly noticeable about the perimeter of the glass where peripheral portions of the glass are in direct contact with the shaping rail.
It would be advantageous to be able to reduce the localized high stresses resulting from the non-uniform cooling to the glass.
2b. Patents of Interest
U.S. Pat. Nos. 3,264,082 to Golightly and 4,687,501 to Reese disclose the use of ballast and lightweight bending iron shields for glass sheet bending molds.
U. S. Pat. Nos. 3,976,462 to Sitara and 4,375,978 to Reese et al. disclose bending iron molds used in sag bending operations. Each includes contoured shaping rails supported by posts on a main support frame that is conveyed through a heating lehr. Flat glass sheets are positioned on the shaping rail of the bending mold and sag into contact with the rails as the glass sheets are heated in the lehr.